Parents are protective of their offspring. They will personally defend you against anyone who tries to hurt you, and they will hold your hand as you cross the street. Some parents will even call you five times a day to make sure you’re eating something other than instant ramen. But parents also equip their offspring to defend themselves.
The seven-spot ladybug (Coccinella septempunctata), for example, contains varying levels of chemicals called precoccinellines and coccinellines at each stage of its lifecycle—egg, larva, and adult. These chemicals are unpleasant to the taste and often toxic to predators, and the conspicuous coloring at each stage acts as a warning signal to predators of this toxicity (this is known as aposematism). This is especially beneficial to ladybugs while they are still eggs. Eggs are often under high predation because they contain so many nutrients for development, which means they also make a really good meal for animals like ants and spiders. This duo of egg defenses is often attributed to maternal investments of carotenoid pigments and toxins into the eggs.
A study recently published in the journal of Functional Ecology investigated the role of these motherly investments of egg carotenoids and toxins in the development and survival of offspring. Do they only aid in defense, or do they have multiple functions? Contrary to the researchers’ predictions, there was no significant relationship between carotenoid concentration in the eggs and the body mass, development time, or survival rate of the adults once they emerged. They did, however, conclude that egg carotenoids do contribute to the aposematic coloration of eggs and eventually the coloration of adults.
The team of researchers also investigated whether these aposematic signals in seven-spot ladybugs are an honest indication of the strength of their chemical defenses. In other words, they tested whether adult ladybug coloration could be used to predict toxin concentrations in adults. Following their hypothesis, the results showed that as the hue (in this experiment this meant the red coloration) of the adults intensified, bodily concentrations of coccinelline also increased. When honest signaling was investigated in the eggs of seven-spot ladybugs, researchers found that there was again a positive correlation between the hue (in this experiment this meant the more yellow or orange coloration) of the eggs and the concentrations of precoccinelline.
These results suggest that the warning signaling in seven-spot ladybugs is an honest signal of its strength of toxicity. However, it appears that different stages of the ladybug lifecycle exploit different toxins and express different coloration. The yellow coloration of eggs correlated with maternal inputs of precoccinelline. The red coloration of adults correlated with coccinelline levels. These maternal inputs of precoccinelline are transformed into coccinelline during development. But it is unclear whether precoccinelline deters predators on its own or if it is just the initial form of coccinelline, which acts as the major toxin. Either way, this suggests that the more a mother invests in her eggs, the more likely the offspring will be protected chemically and visually once they emerge as adults.
Not too long ago it was thought that aposematic signals were only a marker of the presence of toxicity, and that it showed no indication of chemical strength. Although many previous studies have contradicted this belief, the current study is the first (that we know of) to investigate whether there is honest signaling in aposematic eggs. In future studies, it will be interesting to see how the visual systems of different predators pick up on the variation in egg color and adult color, and to see whether coloration has evolved to deter specific predators.
Reference: Winters, A.E., Stevens, M., Mitchell, C., Blomberg, S.P., and J.D. Blount. 2014. Maternal effects and warning signal honesty in eggs and offspring of an aposematic ladybird beetle. Functional Ecology.